Les granites varisques du Massif Armoricain - HAL - INSU

Zone Sud-Armoricaine, renferment les mêmes sources fertiles
métasédimentaires à l'origine de certains granites varisques. Au contraire,
dans chacune de ces zones les roches basiques et intermédiaires sont
spécifiques et proviennent de source mantelliques différentes. Tout se passe
comme si pour chaque zone, la croûte continentale comportait des
caractéristiques communes, alors que le manteau lithosphérique serait
différent.
MOTS CLES : CHAINE VARISQUE, BRETAGNE, PLUTONISME, GRANITES, ASSOCIATIONS
MAGMATIQUES, MAGMATISME JUVENILE, RECYCLAGE.
ABSTRACT. A voluminous intrusive magmatism was emplaced between the end of the
Devonian and beginning of the Permian in the Variscan belt of the Armorican
Massif. We recognize here 5 plutonic suites, different in nature and origin,
using new petro-geochemical criteria.
1) An orogenic calc-alkaline suite highly variable in composition, from
gabbros to biotite-hornblende granites, produced by partial melting of an
enriched mantle source above a subduction zone.
2) A magneso-potassic, calc-alkaline and metaluminous suite, composed of
(a) mafic to intermediate rocks, and (b) biotite ± hornblende porphyritic
monzogranites. Both types could be totally mantle- derived but contamination
and mixing processes with crustal granites are likely.
3) A Fe-K alkaline to calc-alcaline and metaluminous suite composed of (a)
mafic to intermediate rocks and (b) mainly of biotite ± hornblende
monzogranites and syenites. According to isotopes, the mafic rocks are
contaminated mantle rocks and felsic rocks are I-type crustal granites mixed
with mantle material.
4) A magneso-potassic, calc-alcaline and strongly peraluminous suite is
composed of (a) mantle derived monzodiorites and (b) monzogranites and
biotite-cordierite granites, mainly produced from partial melting of metasedimentary
sources with a high rate of melting. We observe frequent mixing
between monzodiorites and granites.
5) A leucocratic magneso-potassic, calc-alcaline and strongly peraluminous
suite consists of muscovite-biotite leucogranites with various texture and
modal composition. They were produced by a low partial melting rate of
metasediments, probably in relation with thermal relaxation processes. Raremetal
granites are related to this suite.
Mafic to intermediate rocks are found in four of the five magmatic suites. A
juvenile mantle input clearly occured during crustal growth processes of the
Variscan belt, but to mostly the Variscan Armorican plutonism derives from
metasedimentary sources at different levels in the continental crust and from
igneous sources in its lower part.
It is interesting to notice that these Armorican intrusive rocks mostly consist
of magmas of different nature and origin emplaced at the same time in a
single massif.
Finally, the distribution of the various suites gives some indications about
composition of the lithosphere. The three main domains of the Armorican
Massif (north, central and south) all contain similar metasedimentary fertile
sources from which formed some Variscan granites. In contrast, in each of
these domains, mafic and intermediate rocks are specific and derived from
different mantle sources. It seems that in each domain, the continental crust
was similar whereas the lithospheric mantle was different.
KEY WORDS : Variscan belt, Brittany, plutonism, granites, magmatic suites, juvenile
magmatism, recycling
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